The accumulation of greenhouse gases such as carbon dioxide in the atmosphere is known to contribute to global warming due to the greenhouse effect. Reducing greenhouse gases in the atmosphere remains a continuing global concern. Unfortunately, despite efforts at reducing carbon dioxide emissions, carbon dioxide concentrations in the atmosphere continue to rise annually primarily due to fossil fuel combustion. The United States Environmental Protection Agency (EPA) estimates that the global atmospheric concentrations of carbon dioxide were 35% higher in 2005 than they were before the Industrial Revolution.
Reducing carbon dioxide emissions has traditionally focused on either reducing fossil fuel combustion or sequestration of carbon dioxide. Sequestration of carbon dioxide is the process of removing carbon from the atmosphere and depositing it in a reservoir. It is a geoengineering technique for long-term storage of carbon dioxide or other forms of carbon to either mitigate or defer global warming. By capturing carbon dioxide as a by-product in processes related to petroleum refining or from flue gases from power generation, the carbon dioxide may be sequestered in this way for long term storage in permanent artificial reservoirs such as subsurface saline aquifers, reservoirs, ocean water, aging oil fields, or other carbon sinks.
Another way of taking advantage of carbon dioxide production is by converting the carbon dioxide to a higher value product. Methanation reactions are one example of a reaction process for converting carbon dioxide to a more desirable product, in this case, methane. Although carbon dioxide may be reacted to produce higher value products, such processes have traditionally been uneconomical due to the low reaction yields inherent in such processes and inefficiencies of current reaction methodologies. Conventional carbon dioxide methanation processes generally require high temperatures to achieve reasonable yields and consequently result in high energy usage. The high reaction temperatures also result in high capital equipment investment for conventional methanation processes. Thus, conventional carbon dioxide methanation processes are plagued with low efficiencies and high costs.
The proposed U.S. Federal cap and trade legislation may further support the economics of carbon dioxide capture and sequestration or alternatively processes that convert carbon dioxide to useful products. Where emission credits are offered for the capture of carbon dioxide, these emission credits enhance the economics of converting carbon dioxide to a more valuable product.
Methanation is typically accomplished through the conversion of carbon monoxide over a conventional nickel catalyst to methane as described by the following chemical reaction:CO+3H2→CH4+H2O
The chemical reaction of carbon dioxide to methane is depicted as follows:CO2+4H2→CH4+2H2O
Achieving desirable reaction in methanation reactions typically requires temperatures exceeding approximately 230° C. using conventional catalysts. This high temperature means that reaction vessels for these reactions must be fabricated out of metallurgies able to withstand the high temperatures or alternatively, one must stage the reaction over multiple reactors in series. In other words, the high temperatures required to achieve economically satisfactory completion of the methanation reactions essentially require either higher capital costs or higher operating costs. The high capital costs are due to having to use reactor metallurgies capable of withstanding the higher temperatures involved or having to stage multiple reactors in series. Where such higher temperatures are avoided by additional cooling equipment, higher operating costs are necessarily incurred.
Another disadvantage of conventional catalysts is the higher coke formation inherent in the use of these conventional catalysts. Catalyst deactivation via coke deposition occurs with any carbon-containing source when oxygen is not present in the stream. The rate of coke deposition is strongly dependent on reaction temperature with higher deposition rates at higher temperatures. Operation at lower temperatures favors slower deposition rates, hence, less deactivation.
Thus, conventional catalysts are deficient in that they lack the ability to satisfactorily complete methanation reactions at sufficiently low temperatures. Consequently, conventional catalysts currently available for methanation reactions fail to realize satisfactory economic results.